1. Technical Field
The present disclosure relates to systems and methods for optical diagnostics in a Proton Exchange Membrane (PEM) fuel cell using absorption spectroscopy.
2. Background Art
Proton exchange membrane (PEM) fuel cells generate electricity directly through two electrochemical reactions. These reactions take place at the interface between a proton conductive membrane and catalyst electrodes. In a PEM fuel cell, controlled hydration of the membrane is required to ensure effective operation. The hydrogen and oxygen feed streams are typically hydrated to bring water vapor into the cell. Several transport processes are responsible for non-homogeneous distribution of water across the cell cross-section including diffusion due to partial pressure gradients and electro-osmotic drag of water by protons through the membrane (see, e.g., J. Larminie, A. Dicks, Fuel Cell Systems Explained, Wiley, New York, 2000, pp. 1-6 and pp. 69-77; and P. Costamagna, S. Sirinivasan, J. Power Sources, 102 (2001) 253-269). In addition, the cathode reactions also produce water that may condense depending on local temperature and partial pressure with respect to the fuel cell.
Overall performance of the fuel cell can be very sensitive to water management since excessive water can lead to flooding and limit the rate of reactant transport to the electrodes. Moreover, a reduction in water can decrease the protonic conductivity of the membrane. Nafion©, which is currently the most common membrane material exhibits a protonic conductivity change of an order of magnitude due to variation of relative humidity between 35 and 85% (see, e.g., G. Alberti, M. Casciola, L. Massinelli, B. Bauer, J. of Membrane Sci., 185 (2001) 73-81). Similarly, the temperature of a PEM fuel cell impacts performance of the catalyst electrodes, water transport and liquid/vapor balance.
Understanding of the distribution of water and local temperatures within operating fuel cells can significantly impact optimization of fuel cell system operation and design. Accurate, fast, in-situ measurements of water concentration would enable both better understanding of water transport, thereby improving cell design and advanced control strategies.
Development of tools for sensing temperature and chemical species in fuel cells is a relatively new area of research. Until recently, most measurements in fuel cell systems were limited to global measurements of electrical cell performance. Polarization curve measurements, for example, are routinely used to track cell performance and can be combined with simple models to diagnose component problems in the cell (see, e.g., M. L. Perry, J Newman, E. J. Cairns, J. Electrochem. Soc., 145 (1998) 5-15). More recent refinement of global measurement techniques has permitted monitoring of flooding or drying conditions based on pressure drop across the cell (see, e.g., F. Barbir, H. Gorgun, X. Wang, J. Power Sources, 141 (2005) 96-101; and W. S. He, G. Y. Lin, T. Van Nguyen, AIChE J., 49 (2003) 3221-3228), separation of anode and cathode contributions to cell polarization based on impedance spectroscopy (see, e.g., J. T. Mueller, P. M. Urban, J. Power Sources, 75 (1998) 139-143), and diagnosis of gas diffusivities at electrodes based on rapid gas supply interruption (see, e.g., J. Stumper, H. Haas, A. Granados, J. Electrochem. Soc., 152 (2005) A837-A844). However, these techniques are generally limited to providing only information integrated across the cell.
More recent developments have enabled characterization of local cell conditions. The development of segmented fuel cells enables measurements of local electrical performance (see, e.g., J. Stumper, S. A. Campbell, D. P. Wilkinson, M. C. Johnson, M. Davis, Electrochem. Acta, 43 (1998) 3773-3783; and M. M. Mench, C. Y. Wang, M Ishikawa, J. Electrochem. Soc., 150 (2003) A1052-A1059). Observation of local chemical conditions have been made using simple visual observations of bubble formation through windowed direct methanol fuel cells (see, e.g., H. Yang, T. S. Zhao, Q. Ye, J. Power Sources, 139 (2005) 79-90), physical probe measurements using gas chromatography (see, e.g., Q. Dong, J. Kull, M. M. Mench, J. Power Sources, 139 (2005) 106-114; and M. M. Mench, Q. L. Dong, C. Y. Wang, J. Power Sources, 124 (2003) 90-98), and more sophisticated optical approaches such as liquid water measurements via neutron scattering (see, e.g., D. Kramer, J. Zhang, R. Shimoi, E. Lehmann, A. Wokaun, K. Shinohara, G. G. Scherer, Electrochem. Acta, 50 (2005) 2603-2614; and R. Satija, D. L. Jacobson, M. Arif, S. A. Werner, J. Power Sources, 129 (2004) 238-245), membrane hydration via x-ray scattering (see, e.g., V. R. Albertini, B. Paci, A. Generosi, S. Panero, M. A. Navarra, M. di Michiel, Electrochem. Sol. State Let., 7 (2005) A519-A521), catalyst composition via x-ray absorption (see, e.g., R. Viswanathan, R. Liu, E. S. Smotkin, Rev. Sci. Instrum., 73 (2002) 2124-2127; and A. E. Russell, S. Maniguet, R. J. Mathew, J. Yao, M. A. Roberts, D. Thompsett, J. Power Sources, 96 (2001) 226-232), Fourier transform infrared (FTIR) spectroscopy (see e.g., I. Tkach, A. Panchenko, T. Kaz, V. Gogel, K. A. Friedrich, E. Roduner, Phys. Chem. Chem. Phys., 6 (2004) 5419-5426), and membrane water content and acidity via fiber based fluorescence (see, e.g., Y. P. Patil, T. A. P. Seery, M. T. Shaw, R. S. Parnas, ACS Fuel Chem. Pre., 49 (2004) 683; and Y. P. Patil, T. A. P. Seery, M. T. Shaw, R. S. Parnas, Ind. Eng. Chem. Res., 44 (2005) 6141).
Most of the techniques available for local measurements of chemical composition are limited by either requiring extractive sampling as in the case of gas chromatography and FTIR spectroscopy, which limits their temporal response, or by using facilities that are not easily implemented in routine system measurements, as in the case of neutron scattering and x-ray absorption. Transient gas-phase measurements using non-intrusive laser-based in-situ diagnostics during a dynamic cycle of fuel cell operation currently do not exist.
Existing water vapor partial pressure measurements related to fuel cell operation are confined to probe sample extraction and inlet and outlet measurements using gas chromatography and Fourier transform infrared spectroscopy. These measurements provide only integrated values across a fuel cell and do not provide local measurements. Accordingly, a need exists for systems and methods for convenient and in-situ gas phase concentration measurement of an operating fuel cell, particularly related to PEM fuel cells.
These and other disadvantages and/or limitations are addressed and/or overcome by the systems and methods of the present disclosure.